Over the years various medical devices, including stents and occluders, have been developed for placement within a preselected internal passage, opening, or defect of a patient. Other complex devices may be delivered and used in treating specific abnormal conditions, such as devices used in removing vascular occlusions or devices used in treating septal defects and the like. Through the advancement in a variety of devices, these stents and occluders have been delivered non-surgically. Certain intravascular devices, such as catheters and guide wires, may be used to deliver these medical devices to a specific location within a patient, including the patient's heart. For example, a catheter may be used to reach a selected coronary artery within the vascular system or a catheter and/or guidewire may be used to deliver a device to an interior chamber of the patient's heart.
Prior to delivering the particular medical device, the size of the internal passage, opening or defect must be determined in order that an appropriately sized device may be provided. The determination of the "stretched diameter" of the opening or defect is desirable to provide a preferred fit between the medical device and the surrounding tissue. In the past, physicians have utilized a balloon catheter in an attempt to determine the size of the internal opening or defect. Typically, the physician will position the balloon within the opening and slowly inflate the balloon, pushing or pulling the balloon of the catheter fore or aft until the physician feels resistance against the balloon. The size of the balloon corresponding with the size of the opening is then determined. The technique of pulling or pushing a balloon catheter through the opening or defect is unreliable and does not determine the size of the opening when the surrounding tissue is stretched. Further, the longitudinal axis of the balloon may not align with the longitudinal axis of the defect. If the longitudinal axis of the balloon and defect are not close in alignment, an erroneous measurement of the size of the defect may be made.
A balloon catheter and a calibrated guidewire having radiopaque regions of known length, may be utilized by a physician during a preliminary fluoroscopic procedure to estimate the defect's size, shape and thickness of the septal wall near the defect. Although useful, the defects exact size when stretched and the shape cannot be determined, thereby increasing the possibility of leakage around the device when the size of the defect is estimated smaller than the actual size of the defect.
Echocardiography has also been used to estimate the diameter of the opening or defect, however, echo measurements are always significantly smaller than the "stretched diameter" of the defect. It is desirable to determine a stretched diameter of the defect, without tearing or otherwise negatively affecting the opening. The differences between echo measurements and stretched diameters may range between 2 mm to 10.5 mm. It has been suggested that the stretched diameter can be estimated from echo measurements by multiplying 1.05 times the echo measurement and then adding 5.49. Although this formula may prove sufficient in some cases, differences of up to 4.5 mm between the actual stretched diameter and the estimated diameter from this formula have been observed. The errors in echo measurements can be explained by the fact that most communications are not perfectly round whereas the balloon transforms the deformed communication into a round structure. If a device is selected which is too small, the risk of embolization and residual shunting increases significantly. On the other hand, if the device is too large, the device may not fit properly within the opening or defect. Thus, there is a need for a device that non-intrusively yet accurately determines the size of the defect opening when the tissue surrounding the opening is stretched.
Other methods have been described for determining the size of the internal opening utilizing a balloon catheter. For example, Taheri et al. in U.S. Pat. No. 5,591,195 describes a sizing catheter, wherein the pressure within an inflatable balloon is measured. Taheri et al. teaches that when the balloon makes contact with a vessel to be measured, the pressure within the balloon increases. The size of the balloon may then be determined from a chart of known balloon pressures and diameters. As seen in FIG. 9, the pressure within the balloon may vary even though the actual diameter being measured remains the same. A need therefore exists for a device that can determine the stretched diameter of an internal passage, opening or defect. The present invention meets these and other needs which will become apparent to those skilled in the art.